Logout: Allowing the handling of the url parameter to be altered.

The CAS protocol specifies the following:

    2.3.1. parameters

    The following HTTP request parameter MAY be specified to /logout. It is case
    sensitive and SHOULD be handled by /logout.

    * url [OPTIONAL] - if "url" is specified, the URL specified by "url" SHOULD
      be on the logout page with descriptive text. For example, "The application
      you just logged out of has provided a link it would like you to follow.
      Please click here to access http://www.go-back.edu."

That default behavior is nice for a pure auth server, but in practice it's nice
to forward users to the service whence they logged out.

This commit therefore adds a new setting called `MAMA_CAS_FOLLOW_LOGOUT_URL`. If
its value is `True`, then instead of displaying a message to the user we provide
the logout `url` value to login as a `service` parameter.